For being able to create actual 3D shapes and polygons, I would recommend that you check out Google Sketch, it’s free and pretty easy and fun to use. (Not 100% sure that Google Sketch is it’s correct name, it’s been a while since I deleted it, I had no need for it.)

Illustrator would be perfect 2D illustrations. For quick, rough 3D drafts, Sketchup Pro would be a good start you can import your 2D file in there. For realistic renderings Maxwell Render is a really good app.

For a slick little vector design program that doesn’t cost a lot, check out Millwrite. It’s primarily for CNC layouts but I used it for furniture design and sketches. Has a user friendly interface, great features, and reasonable price.

I’m sure with a bit of searching you can find a demo program so you can try before you buy.